Контрольные работы по английскому языку.
Воскресенье, 16.06.2019, 09:57


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The question is often asked: "What is an electric current?" No one has ever seen it. We only know of the existence of a current owing to its effects. A current can heat a conductor, it can have a chemical action when passing through a solution, or it can produce a magnetic effect. We can measure currents by observing their heating, chemical or mag­netic effects. The practical unit of current is called the Ampere.

Two things are necessary to cause an electric current to flow: first - a complete circuit, and second - a driving force called the elec­tromotive force (e. m. f.).

If you put free electrons on an insulated copper ball, what would they do? In this case they would try to repel each other. In case you connected this charged ball to another ball of equal size by a copper wire, what would be the result? The electrons would move along the copper wire un­til the numbers of electrons on each ball were the same. This is an exam­ple of electromotive force causing a current to flow.

A battery has a surplus of electrons on one of its two plates; so you say that a battery furnishes an e. m. f. If a copper wire is run from one plate to the other, а current flows in the complete circuit thus made. If a small bulb is placed in the circuit, it will light up, giving evidence to a current flow. If the battery were disconnected and a generator substituted for it, we should have a typical lighting system. Both batteries and gen­erators are the most common sources of electromotive force. The practi­cal unit of e. m. f. is the Volt.

        Currents will flow more readily in some substances than in others, that is, various substances offer lesser or greater resistance to the flow of current. Such substances as porcelain, ebonite, rubber, glass and the like having extremely high resistance are known as insulators. The practical unit of resistance is the Ohm.

        Substances whose properties lie between those of conductors and in­sulators are called semiconductors. Let us name but a few most widely used at present, they are germanium, silicon, selenium and copper oxide. The importance of semiconductors in our life cannot be overestimated. But for these tiny "workhorses" electronic industry would not have achieved such a great progress.

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Контрольные работы по английскому языку.